We propose to study fibroblasts from hereditary retinoblastoma (HRB) patients compared with sporadic retinoblastoma and unaffected controls to identify cellular and molecular factors which may predispose HRB individuals to malignancy. We will examine x-ray survival to test the hypothesis that a cellular defect in DNA repair renders a subpopulation of HRB patients at increased risk of developing second (non-ocular) tumors. Prophase chromosome banding technqiues will be used to amplify the q14 region of chromosome 13 in an attempt to detect putative microscopic deletions involved in the etiology of retinoblastoma as well as in the proposed DNA repair deficiency. We will undertake parallel biochemical studies to correlate radiation sensitivity with capability of repair of gamma-radiation-induced and chemical-induced damage. Concommitant studies will investigate spontaneuos, x-ray-induced and chemically-induced mutability of HRB versus normal fibroblasts, as well as any diffrential extents of chromosome breakage and sister chromatid exchange. These detailed characterizations of HRB fibroblasts at the chromosome, gene, and molecular levels are designed to aid in the elucidation of mechanisms involved in retinoblastoma carcnogenesis as well as to provide bases for genetic counseling of affected families and for prediction of probabilities of second tumor development in affected individuals.